The log mean temperature difference (LMTD) is used to determine the temperature driving force for heat
transfer in flow systems.
The LMTD is a logarithmic average of the temperature difference between the hot and cold streams at each end of the exchanger.

This calculator is used to calculate LMTD for
parallel flow (i.e. where the hot stream,liquid or gas, goes from say left to right, and so does the cold stream).

The larger the LMTD, the more heat is transferred.
The use of the LMTD arises straightforwardly from the analysis of a heat exchanger with constant flow rate and fluid thermal properties.